Powder metallurgical technology is capable of producing machine parts having complicated shapes with high dimensional precision and is thus capable of significantly decreasing the production costs of the machine parts. Therefore, various machine parts produced by applying the powder metallurgical technology are used in many fields. Further, in recent years, the requirement for miniaturization or weight lightening of machine parts has increased, and various raw material powders for powder metallurgy for producing small and lightweight machine parts having sufficient strength have been investigated.
For example, Japanese Unexamined Patent Application Publication No. 1-219101 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2-217403 (Patent Document 2), Japanese Unexamined Patent Application Publication No. 3-162502 (Patent Document 3), and Japanese Unexamined Patent Application Publication No. 5-148505 (Patent Document 4) disclose raw material powders for powder metallurgy produced by adhering an alloying powder to surfaces of a pure iron powder or alloy steel powder with a binder (referred to as “segregation-free treatment”). Such powders mainly composed of iron (in a narrow sense, referred to as an “iron-based powder” hereinafter) are usually produced by adding an additive powder (e.g., a copper powder, a graphite powder, an iron phosphide powder, a manganese sulfide powder, or the like) and a lubricant (e.g., zinc stearate, aluminum stearate, or the like) and the resultant mixed powders (also referred to as “iron-based powders” in a broad sense) are supplied to production of machine parts. Hereinafter, the iron-based powder has a broad sense unless otherwise specified.
However, the iron-based powder (narrow sense), the additive powder, and the lubricant have different characteristics (i.e., the shape, particle size, and the like), and thus flowability of a mixed powder is not uniform. Therefore, the following problems occur:
(a) The iron-based powder (narrow sense), the additive powder, the lubricant, and the like locally unevenly distribute due to the influence of vibration or dropping during transport of the mixed powder to a storage hopper. The deviation due to differences in flowability cannot be completely prevented even by the segregation-free treatment.
(b) Since relatively large spaces are produced between particles of the mixed powder charged in the hopper, the apparent density of the mixed powder decreases.
(c) The apparent density of the mixed powder depositing in a lower portion of the hopper increases over time (i.e., due to the influence of gravitation), while the mixed powder in an upper portion of the hopper is stored at a low apparent density. Therefore, the apparent density of the mixed powder is nonuniform in the upper and lower portions of the hopper.
It is difficult to mass-produce machine parts having uniform strength using such a mixed powder.
In order to solve the above problems (a) to (c), it is necessary to increase flowability of the mixed powder of the iron-based powder (in a narrow sense), the additive powder, and the lubricant.
Therefore, Japanese Unexamined Patent Application Publication No. 2002-180103 (Patent Document 5) discloses an iron-based powder mainly composed of an iron powder having a predetermined range of particle diameters. However, this technique not only decreases the yield of the iron powder because an iron powder out of the specified range cannot be used but also causes difficulty in uniformly and sufficiently filling thin-walled cavities, such as a gear edge or the like, with the ion-based powder.
In addition, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-515542 (Patent Document 6) discloses a technique for improving flowability of an iron powder in warm compaction by adding a small amount of inorganic particulate oxide (e.g., 0.005 to 2% by mass of SiO2 having a particle diameter of less than 40 nm) having a particle diameter of less than 500 nm (nanometer). However, in this technique, an oxide such as SiO2 remains in sintering and inhibits bonding between iron powder particles, thereby decreasing strength of the resultant sintered body.
Further, PCT International Publication No. WO06/004530 A1 (Patent Document 7) discloses a powder metallurgical composition containing an iron powder or an iron-based metal powder, a lubricant and/or a binder, and carbon black as a flowability increasing agent, the amount of the carbon black being 0.001 to 0.2% by weight. This technique is deemed to be not associated with deterioration of quality of sintered parts.
As the iron powder or alloy steel powder used as a raw material of the iron-based powder, there are an atomized iron powder, a reduced iron powder, and the like according to the production methods. Here, a pure iron powder may be referred to as an iron powder, but the term “iron powder” in the classification by production methods is used in a broad sense including an alloy steel powder. Hereinafter, the term “iron powder” represents an iron powder in the broad sense. The alloy steel powder includes steel powders other than prealloys, i.e., a partially alloyed steel powder and a hybrid alloyed steel powder.